1. Field of the Invention
The present invention relates generally to systems for electrophotographically reproducing data on photosensitive film, and more particularly, to an improved copyboard upon which documents are placed for copying the information in the document.
2. Description of the Prior Art
Stimulated by the cost incurred in storing the ever increasing volume of documentary material which is presently being generated, considerable effort has been expended in the design and development of various electrophotographic and other data storage and retrieval systems. Particular emphasis has been placed on the design of microphotographic systems which may be used to decrease the physical bulk of such storage materials without sacrificing file retrieval and/or copy reproduction capabilities. One type of apparatus for recording data on film for data storage is the indirect type of reproduction apparatus wherein a document is placed face down on a transparent copyboard and a light image is reflected from the document and directed by mirrors through a lens system and onto the film. This type of apparatus is disclosed in U.S. Pat. application Ser. No. 349,452, entitled Electrophotographic Method and Apparatus and filed Apr. 9, 1973 in the name of Frank C. Gross.
Electrophotographic film suitable for use in the apparatus described in U.S. Pat. application Ser. No. 349,452 are well known and typically consist of a support layer having coated thereon a layer containing a suitable photoconductor. The support layer is made conductive by either the inclusion therein of electrically-conductive materials or by coating the support layer surface designed to receive the photoconductive layer with an electrically-conductive material. Images are formed on the photoconductive layer within the above-described reproduction apparatus by first applying uniform electrostatic charge to the photoconductive layer and thereafter imaging the charged photoconductive layer by exposing it to light reflected from the document being reproduced, thereby causing the photoconductive layer to become conductive, which results in the dissipation of the charge in those areas of the layer exposed to light. In a subsequent step, the charge pattern or latent image on the photoconductive layer is rendered visible by the application thereto of colored or black electroscopic toner particles.
A major difficulty in prior art system is that, to produce a visible image of sufficient intensity, undesirable light and dark areas are sometimes present due to distortion of the electric lines of force on the charged surface of the photoconductive layer. Prior efforts to reduce this effect have been through the use of a development electrode spaced from the surface of the film being developed and connected to a source of electric potential to provide a bias that affects the field about the charged photoconductive layer and causes the charged developer particles to migrate to the highest or lowest charged regions (depending upon polarity). Other efforts have employed the development electrode in an electrically floating mode.
Charging is conventionally accomplished by exposing the surface of the photoconductive layer to a corona discharge, the polarity of which is chosen to produce the desired results upon the particular photoconductive layer being charged. Superior image reproductions are obtainable only when very uniform electrostatic charges are established on the photoconductive layer before imaging. In some electrophotographic imaging apparatus either the corona generating element or the electrophotographic recording element is moved during charging, which to some extent improves uniformity of charge over the surface of the photoconductive layer. In other electrophotographic apparatus (the apparatus of U.S. Ser. No. 349,452 being an example), charging takes place with no relative movement between the corona generating element and the electrophotographic recording medium. In such cases, the recording element may be a multi-frame microfiche and charging is commonly restricted to a small layer on the electrophotographic member by some form of shielding or masking means. This form of charging is accomplished without relative movement between the member and the charging means, and it results in a generally uniform potential of several hundred volts across most of the surface being charged and a potential of zero volts at the borders of the area being charged. Unfortunately, the portion of the surface having uniform charge does not extend up to the borders. Rather, the amount of charge tapers down to zero volts over some finite distance as the borders are approached. After imaging and developing the charged area, this border area has undesirable edge toning because of the charge gradient occuring there. Where the imaging step dissipates the entire charge in the border region, edge toning is not such a problem, but in conventional apparatus the charge in the border region is seldom entirely dissipated.
The prior art has dealt with this and related problems in several ways. For example, U.S. Pat. No. 3,556,655 to Lux et al. discloses discharge of the border regions with special exposure lamps to prevent any toning of those areas. U.S. Pat. No. 3,512,965 to Matkan discloses elimination of toning in the non-imaged areas, which includes both the border areas and the areas between the image lines, by subjecting the exposed electrophotographic member to a coating of a barrier chemical to reduce the tendency of the toner particles to adhere to the areas which are are not highly charged. U.S. Pat. No. 3,671,121 to Albert is concerned with the undesirable black border created by making positive copies from negatives, and discloses elimination of the black border by imaging through an opaque border mask. U.S. Pat. No. 3,687,538 to Matsumoto discloses elimination of undesirable black border by use of a discharging light source at the borders. All of these prior art devices require complex apparatus, which obviously is undesirable if simpler alternatives can be found. Therefore, it is the object of the invention to eliminate undesirable edge toning on an electrophotographic member through use of a simple, but effective, apparatus.